In a hot rolling process in a process of production of steel sheet/strip, steel plate, or other ferrous metal products, lubricated rolling is performed for lightening the load on the rolling rolls which are used as working tools, reducing wear or seizing and the occurrence of defects caused along with the wear or seizing, securing good surface quality of the products, and various other purposes.
In the hot rolling process, mainly, the method of using the water injection method to spray and supply a mixture of water and lubricant in an emulsion state (for example, see NPLT 1) to the roll, and the method of supplying grease or other semisolid lubricants by air or another gas to deposit it on the rolls, etc. have been used (for example, see PLT 1).
As other lubricated rolling methods, the method of directly pressing a solid form lubricant, which is made by mixing graphite or another solid lubricant with wax, to the roll surface (for example, see PLT 2), and the method of supplying a non-oil type lubricant, which is made by mixing various additives with a colloidal solution, to the rolls or roll bite, etc. are also known.
Further, in recent years, as a method of supplying a lubricant not using water, the method of rendering, not a semisolid grease, but a liquid lubricant which is used in the water injection method into an atomized or particulate state and spraying and supplying this to a roll together with a noncombustible gas has been proposed (PLT 3, below, this method called the “gas atomization method”). According to this method, a small amount of lubricant supply enables a large effect of reduction of the coefficient of friction to be obtained. Furthermore, equipment for lubrication and a method of lubrication for using the gas atomization method in hot rolling of steel sheet/strip have also been proposed (for example, see PLT 4).
On the other hand, to supply a sufficient amount of lubricant to the parts in which lubrication is required during rolling, usually a rolling mill is provided with a plurality of spray nozzles for lubricant supply. A hot rolling mill for steel sheet/strip, as illustrated in PLT 1 or PLT 4, is provided with a lubrication header comprised of a plurality of spray nozzles aligned in the product width direction. Such a configured lubrication header is designed to enable a lubricant to be supplied to the entire region where a roll and steel material contact each other. This lubrication header, while differing depending on the size of the rolling mill, has at least two spray nozzles which are set at substantially equal intervals and in accordance with need is provided with a mechanism which enables selection of the spray nozzles to be used.
Such a lubrication header is usually individually provided for each of the upper and lower work rolls and backup rolls. However, depending on the operating conditions of the rolling processes, sometimes it is set for just one of the upper and lower work rolls or backup rolls. In either case, when using such a lubrication header for lubricated rolling, at the present, at least two spray nozzles are used in one rolling mill for each pass so as to supply the lubricant.
Regarding the device for feeding a lubricant to the spray nozzles, in the water injection method, as shown in FIG. 1, water and a lubricant are fed by separate pumps, that is, a water-feed pump device 3′ and a lubricant-feed pump device 3, to a mixer of water and lubricant called an “injector 8”. The amount of fed water and lubricant at this time are set so that the emulsion which is produced at the injector 8 becomes a predetermined concentration. The emulsion of the predetermined concentration which is produced at the injector 8 is fed through pipes which are branched in the span from the injector 8 to the spray nozzles 1′ to be fed to the plurality of spray nozzles 1′.
The lubricant concentration of the emulsion is, in hot rolling of steel sheet/strip, 0.2 to 1 vol % or so. The amount of the supplied emulsion as a whole reaches several liters to tens of liters per minute per pass.
To supply such a large amount of emulsion, a relatively high pressure has to be applied to spray it from the spray nozzles. Therefore, the speed of feed of the emulsion in the pipes is large, and thus there is no time for the emulsion to separate into water and a lubricant, so the spray nozzles are fed comparatively evenly with substantially the same amounts of emulsion which are then sprayed to the roll. Therefore, in the case of the water injection method, for convenience in laying the piping, equipment for supplying a lubricant which is provided with two sets of pump devices for upper roll and lower roll is usually used. One set of pump devices is used to feed emulsion to two or more spray nozzles.
Regarding the gas atomization method, equipment for supplying a lubricant and method of supplying a lubricant which use a lubrication header which is provided with so-called internal mixing type two-fluid spray nozzles which are provided with mixing chambers at which the lubricant and gas are mixed inside the spray nozzles are disclosed in PLT 4. With this equipment of supplying a lubricant and method of supplying a lubricant, the lubricant is fed to the spray nozzles as much as possible without pressure in the lubricant pipes. Gas of less than 0.5 bar (0.05 MPa) is used to render the lubricant into a particulate or atomized state and spray and supply it from the spray nozzles.
PLT 6 discloses a method of supplying a lubricant and equipment for supplying a lubricant which are designed to prevent scatter even if floating mist not deposited on the roll is formed when rendering a lubricant to the particulate or atomized state and spraying and supplying it from the spray nozzles by the gas atomization method. In the system described in PLT 6, an air spraying mechanism is provided as a secondary nozzle at the outside of the flow paths of the spray nozzles through which the lubricant is sprayed. At the time of spraying the lubricant, air is blown from the air spraying mechanism to form a wall of air to thereby suppress splatter of the floating mist.